Process for removing chlorates from caustic solutions



Patented July 9, 1940 rnocass roammovmo cmnna'ras momcans-no SOLUTIONS Irving E. Muska t, Akron, Ohio, aslignor to Pittaburgh Plate Glass Company,

Pa a corporation of Pennsylvania Application February 0, loss, Serial No. reason 5 Claims. (01. 28-184) This invention relates to the purification of alkali metal hydroxides such as sodium hydroxide and is particularly directed to the removal of chlorates from solutions ofsuch hydroxides.

Alkali metal hydroxides, such as sodium hydroxide, are initially produced as relatively dilute solutions usually having a concentration of about 10-15 per cent and containing large quantities of impurities, such as alkali metal chlorides, chlorates, carbonates, iron, aluminum and silicon compounds, etc. Since the majority of these impurities have a minimum solubility in solutions of sodium hydroxide of -50 per cent concentration, it has been the practice to concentrate to this concentration and to remove solidified impurities.

While this treatment permits the removal of large amounts of impurities, it does not remove such materials as sodium chloride and sodium chlorate to the degree generally considered desirable. Electrolytic caustic, especially that prepared in a diaphragm-type cell, contains large amounts of these impurities which must be removed in order to secure caustic which has the purity regarded as necessary for many purposes. While sodium chloride may be at least partially removed by suitable crystallization processes involving the precipitation oi certain hydrates of sodium hydroxide the concentration of sodium chlorate does not appear to be completely removed by these processes.

As is commonly recognized, aqueous solutions of sodium hydroxide are corrosive and readily corrode most metals. The presence of chlorate in such solutions results in a manifold increase in the corrosive power of the caustic. Solutions of caustic containing small amounts of chlorate are so corrosive it is practically impossible to maintain them in an uncontaminated condition for any length of time or to store them in usual equipment because of the rapidity with which these solutions corrode metals such as iron. For this reason electrolytic caustic made by the diaphragm-type cell which contains substantial quantities of chlorate has been at a serious disadvantage in competing with caustic prepared by other processes.

In accordance with my invention, I have found that chlorate may be substantially or wholly removed from aqueous solutions of alkali metal hydroxides by treatment in the manner hereinafter described. In U. .8. Patent No. 1,742,220 to George A. Richter, a process is described wherein electrolytic caustic is decolorized and colloidal impurities removed by treatment with fibrous materials containing alpha-cellulose. In general.

treatment in this "process is carried out at room temperature although it is stated that a temperature of 40 C. may be suitable. The process is operated, however, under such conditions or temperature and time of contact that no substantial reaction occurs between the caustic understood by reference to the following descrip tion. illustrated by the acc0mp= 1:; which:

Figure 1 is a graph illustrating the effect of g drawings in alpha-cellulose upon the chlorate content oi sodium hydroxide solution at 100 C. and;

Figure 2 is a graph illustrating the eflect oi alpha-cellulose at 114 C. y

The treatment may be carried out in any convenient manner which will insure the required contact between the alpha-cellulose fiber and the caustic. This may be done, for example, by heating and agitating a dispersion of alpha-cellulose in caustic liquor for the required period of time. In accordance with one very effective method, I may incorporate alpha-cellulose fiber in caustic solution and introduce the mixture into evapo= rators. By this means, I may secure removal of chlorate and concentration of the liquor simultaneously.

As will be apparent from an inspection of I Figures 1 and 2, the amount of chlorate removed appears to depend upon the temperature and concentration of the solution undergoing treatment. These curves illustrate the amount of chlorate removed by a given weight 'of alpha-cellulose after treatment of the liquor with the cellulose for varying periods of time, at fixed temperatures. Figure l illustrates the efi'ect of uniform weights of alpha-cellulose added to 50. 80, and 15 per cent solutions of NaOH, respectively, which contain 0.975 per cent and 0.321 per cent chlorate, respectively, based upon the amount of NaOI-I in the solution at a temperature of 100 0. Note that the amount of chlorate removed from a per cent solution after the first two hours of treatment is small while treatment of solutions of 30 and 15 per cent concentration results in a much more complete removal of this impurity. Figure 2 illustrates a similar trend and donstrates the increase in rapidity and degree of chlorate removal with increased temperatures.

I have found that chlorate may be more completely removed by treatment of relatively dilute solutions at relatively. high temperatures suitably at or near the boiling point of the solution. For most efficient results, solutions not substantially exceeding 40 per cent in concentration may be treated at temperatures not substantially less than C. and preferably in the neighborhood of 100-115" C. Some chlorate may be removed from solutions of higher concentrations as is shown by the accompanying Figures 1 and 2, but treatment of solutions substantially in excess of 35-40 per cent does not appear to result in the marked chlorate reduction noted in the treatment of more dilute solutions. Similarly, chlorate may be removed by operating at temperatures below 100 C. but treatment at temperatures substantially below 70 C. does not appear to result in successful purification within the period of time generally regarded as necessary for commercial scale operation.

The fiber to be employed should preferably be one in which alpha-cellulose is the predominant constituent since, in general, fibers low in alphacellulose contain substances which react with the caustic to produce colored products. Alpha-cellulose derived from a wood pulp refining process or purified cotton fiber is suitable for this purpose. Up to 5 per cent of the alpha-cellulose based upon the amount of NaOH present in the solution being treated may be used in order to insure the presence of an excess.

The solution may be treated for removal of dissolved cellulose if desired, and conventional methods of dialysis may be resorted to for this purpose, Removal of the cellulose may be regarded as unnecessary, however.

The following examples illustrate the invention:

Example I.Ten parts by weight of alphacellulose paper was cut into the shreds and added to one hundred parts by weight of a 30 per cent sodium hydroxide solution containing 0.975 per cent of chlorate on the anhydrous basis. The solution was heated at a temperature of 100 C. under a reflux condenser for 25 hours. The solution after filtration contained substantially-no chlorate.

Example II .-One part by weight of shredded alpha-cellulose was added to one hundred parts by weight of a 15 per cent sodium hydroxide soluaaoaass tion conta anhydrous basis. The solution was heated under a reflux condenser at a'temperature of 114 C. for 6 hours. After filtration, the solution was substantially free from chlorate.

Example III.-One part of shredded alphacellulose was introduced into fifty parts of a 14 per cent solution of sodium hydroxide containing 0.3 per cent chlorate and the mixture introduced into a suitable evaporator and evaporated until the evaporated liquid contained 50 per cent sodium hydroxide. The liquor so produced contained substantially no chlorate.

Although this invention has been described in connection with the specific details of certain embodiments thereof, it is not intended that" such details shall be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.

I claim:

1. The process of removing chlorate from an aqueous solution of alkali metal hydroxides containing the same which comprises contacting said solution with suflicient alpha-cellulose at a temperature upward of about 70 C. to substantially completely remove chlorate.

2. The process of removing chlorate from an aqueous solution of sodium hydroxide containing the same which comprises contacting said solution with sumcient alpha-cellulose at a temperature of not substantially less than 100 C. to substantially completely remove chlorate fromthe solution.

3. A process of removing chlorate from aqueous solutions of sodium hydroxide which comprises contacting an aqueous solution of said hydroxide, the concentration of which does not substantially exceed 40 per cent, with sufllcient alpha-cellulose at a temperature upward of about 70 C. to substantially completely remove chlorate from the solution.

4. The process of removing chlorate from an aqueous solution of sodium hydroxide which comprises-evaporating a relatively dilute solution of said hydroxide and maintaining said solution in contact with suflicient alpha-cellulose during at least a portion of the period of evaporation at a temperature upward of about 70 C. to substantially completely remove chlorate from the solution.

5. The process or claim 4, wherein the evaporation is carried on until a solution of 45-50 per cent sodium hydroxide is obtained.

' IRVING E. MUSKAT.

0.32 per cent chlorate on the 

